1. Field of the Invention
This invention relates to lasers that emit sub-picosecond pulses, and particularly to semiconductor lasers called quantum cascade lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum. These pulses have particular importance to applications where materials must be processed while minimizing heat transfer and damage to the materials, for example tissue removal in medicine and dentistry, and micromachining at the pico and femto scales. These pulses also have importance to applications involving spectroscopy, such as detection of trace compounds, clock synchronization in computer networks and telecommunication networks, and in light detection and ranging (lidar).
2. Background of the Invention
Although interband lasers can be high-power lasers, it is difficult to obtain any power at all in interband lasers in the mid-IR. Quantum cascade lasers are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum. Compared to other semiconductor diode lasers that emit in the mid-far IR, QC lasers have higher output power since laser emission is achieved from the transition of an electron through periodic thin layers of material forming a superlattice that introduces an electric potential over the device. Unlike other lasers, the lattice allows the electron to emit multiple photons as it traverses, or cascades, from one period of the lattice to the next. Semiconductor lattices in QC lasers can be made from layers of crystalline aluminum indium arsenide alternating with indium gallium arsenide, which create structures called quantum wells. One of the interesting features of QC lasers is that they operate at room temperature, without the need for cooling that might be found in non-QC lasers.
A technique for creating extremely short pulses of laser light is called mode-locking. By pumping a laser into a laser cavity consisting of two or more mirrors, the normal random oscillations of the light waves can be made synchronous and thus increase (constructive interference), called mode-locking, or may be made to interfere and damp (destructive interference). By using an electronic device that modulates the light intensity, an electrical signal can be used to establish a short pulse within the laser cavity in a technique called active mode-locking. However, in active mode-locking, pulses that are less than about one picosecond have not been obtained. An actively modelocked QCL that produces 3 ps pulses at 6.2 μm was reported by Wang et al.
In contrast, passive mode-locking does not require an external signal to maintain a continuous stream of pulses. Some examples of passive mode-locking use an intracavity absorber to absorb low-intensity light and preferentially amplify high-intensity spikes. Certain dyes in solution can act as saturable absorbers, but graphite-lattice structures, and the use of an aperture have also been used to focus and amplify the high-intensity light and attenuate the low-intensity light. Aperture transmission of high-intensity light and attenuation of low-intensity is called Kerr lens mode-locking, sometimes called “self mode-locking” However, no semiconductor laser has been passively mode-locked in the mid-IR range.
Laser light can have an interesting relationship to the medium or material that it impacts. McCall and Hahn observed a nonlinear optical propagation effect in resonant medium. According to their observation, a short pulse of coherent light above a critical input energy, for a given pulse width τ<T2, can pass through a saturable resonant medium as though the medium were transparent. However, below the critical energy this self-induced transparency (SIT) phenomenon cannot happen, rather, the pulse energy is absorbed.
Because QCLs typically have relatively short values of T1 (1-10 ps) and relatively long values of T2 (˜100 fs) compared to other semiconductor lasers, standard passive modelocking cannot be achieved. However, the conditions are ideal for any approach, denoted SIT modelocking. After the original observation of passive modelocking, it was speculated that passive modelocking was SIT modelocking, but this speculation was later proved to be incorrect. Still later, Kozlov studied the possibility of obtaining SIT modelocking by using gas lasers with two separate tubes in which the gases in the two different tubes have the same resonance lines and the gas in one tube has twice the dipole moment of the other. However, this design cannot be implemented in practice. Applications for mode-locked QC lasers include three-dimensional micro-, or nano-fabrication or machining, investigative and diagnostic spectroscopy, optical communication and electronic devices such as limiters or storage, and medical and dental probes and surgical devices.